The present invention relates to a method of operating a gas generating system as well as to a gas generating system. In particular, the present invention relates to a method of operating a gas generating system in which a high hydrogen yield is achieved by controlling the mass flow rate of individual educts.
Because of their high efficiency, fuel cells are increasingly used for generating electric energy for stationary as well as for mobile applications. In this case, fuel cells are normally operated by hydrogen. However, the storage of hydrogen is difficult and requires high technical expenditures. For this reason, hydrogen is stored in the form of fuels, such as hydrocarbons and alcohols and, corresponding to the requirement of the fuel cell, and is obtained by a gas generating system. In the gas generating system, the fuel is converted with the addition of air and/or water to a hydrogen-rich gas.
Chemical processes for generating hydrogen from hydrocarbons or alcohols are partial oxidation, vapor reforming, and autothermal reforming. Partial oxidation is an exothermal reaction in which the fuel substoichiometrically reacts with oxygen. As a rule, atmospheric oxygen is used for this purpose. During endothermal vapor reforming, the fuel reacts with water while heat is supplied. Autothermal reforming is a combination of partial oxidation and vapor reforming. In this case, the reaction heat of the partial oxidation is provided for the vapor reforming. However, the reaction is not necessarily carried out autothermally.
German Published Patent Application 24 52 147 discloses an automatic control system for a gas generator in which high-grade working gases are generated from hydrazine in a catalytic exothermal reaction. Since, in the case of many catalyst materials, the efficiency is increased as the temperature rises, a cooling of the catalyst temperature is to be avoided which is caused by an excessive amount of fed cold liquid fuel. For this purpose, a flow control valve is provided in the inflow of the liquid fuel and is controlled by a temperature-dependent control element, specifically a bimetallic strip. The flow control valve is opened up as the temperature rises and closes again when the temperature falls because of an excessive cooling by the liquid fuel.
U.S. Pat. No. 4,400,180 discloses a process for partial oxidation and a control system for a continuous generating of a synthesis gas from a fuel. In this case, the fuel may be replaced during the process by another fuel without the requirement of reducing the system or lowering the pressure in the gas generator. For this purpose, a non-catalytic burner is provided for the partial oxidation with central guiding devices, which are spaced radially from concentric coaxial outer guiding devices. Coaxially ring-shaped passages extend between the two guiding devices. The known control system permits a change-over between different fuel currents which flow either through the central guiding devices or the ring-shaped passages, and an adjustment of the flow rates of the reactant flow of the free-oxygen-containing gas. Water is used as a temperature moderator and, if required, is guided into the reaction zone in order to adjust the atomic ratio of free oxygen/carbon and the weight ratio of water/fuel in the reaction zone according to the conditions for autothermal reforming.
Manual or automatic control devices are provided for switching and controlling the fuel, oxygen, and vapor flows. By these devices, a change-over takes place between basic fuel and spare fuel. The flow of the gas containing free oxygen and the temperature moderator are controlled to maintain an efficient and stable gas emission. For this purpose, the flow rates of the four currents (specifically vapor, basic fuel, spare fuel, and gas containing free oxygen) are determined by individual sensors. The actual flow rates are compared with desired flow rates, and the flow rates are adjusted correspondingly. By controlling the flow rates corresponding to the desired flow rates, the temperature in the burner is kept constant within a temperature range of ±200° C.
U.S. Pat. No. 4,490,156 discloses a control system for a continuous generating of synthesis gas by partial oxidation, in which a fuel can be replaced by another fuel during the process without the requirement of reducing the gas generator or lowering the pressure. A non-catalytic burner, which has two sections and a rapid transition characteristic, is used for implementing the partial oxidation of the fuels. The control system provides for switching the fuel currents which flow in one or both sections of the burner. The flow rates of the oxygen, of the fuel, and of the temperature moderator can be changed in one or both sections of the burner to cause a fast transition of the burner. Separate flow rate sensors and separate flow rate control devices are provided in each feed line for an independent monitoring and adjusting of the flow rates of the individual reaction partners. The temperature and the weight ratio of water/fuel in the reaction zone is controlled by controlling the flow rates of the reactant flow of the gas containing free oxygen with or without a temperature moderator and optionally while adding additional water.
The control systems known from the prior art either have a simple design and a very limited control capacity or have a complicated and high-expenditure construction for permitting a variable control because separate sensor and control devices for controlling the flow rates are provided for each of the reaction partners.
In contrast, it is an object of the present invention to provide a method of operating a gas generating system as well as a gas generating system, in which a high hydrogen yield is ensured by controlling the educt flows by cost-effective and constructively simple control and monitoring expenditures, particularly by a limitation of the number of control and monitoring devices.
Accordingly, a reactor of a gas generating system is operated at least approximately adiabatically. For adjusting and maintaining a desired ratio of the mole flow O2 oxygen/mole flow C-atomsfuel (i.e., O2/C ratio) of the educts,                (1) the temperature of the educts is determined before they are charged into the reactor;        (2) an O2/C ratio for the determined educt temperatures (O2/Ceduct temp.) at which theoretically a conversion of the educts takes place at a desired reaction temperature is determined by comparative values;        (3) the O2/C ratio according to the educt temperatures (O2/Ceduct temp.) is adjusted (a) by controlling the mass flow rate of the oxygen-containing educt for a known defined mass flow rate of the hydrocarbon-containing educt and optionally water or (b) by controlling the mass flow rate of the hydrocarbon-containing educt for a known defined mass flow rate of the oxygen-containing educt and optionally water;        (4) a momentary reaction temperature of the reactor is determined;        (5) a correction value ΔO2/C for the O2/C ratio is determined by comparative values from the temperature difference between the actual reaction temperature and the desired reaction temperature;        (6) the O2/C ratio according to O2/Ceduct temp. and ΔO2/C is changed (a) by controlling the mass flow rate of the oxygen-containing educt in the case of a known defined mass flow rate of the hydrocarbon-containing educt and optionally water or (b) by controlling the mass flow rate of the hydrocarbon-containing educt in the case of a known defined mass flow rate of the oxygen-containing educt and optionally water.        
During partial oxidation as well as during autothermal reforming, the hydrogen yield is influenced by (1) the preheating temperature of the educts at the reactor inlet, (2) the ratio of the mole flow rate O2/mole flow rate C-atomsfuel, for example, the ratio of the supplied air quantity to the fuel quantity, and (3) in the case of a combination of partial oxidation and water vapor reforming, i.e., autothermal reforming, additionally by the supplied water quantity. In this case, the hydrogen yield is highly dependent on the O2/C ratio. Even small deviations from an optimal ratio cause losses in the yield. In an adiabatically-operated reactor, changes in the O2/C ratio cause a considerable change of the reaction temperature. A conclusion can therefore be drawn with respect to the O2/C ratio in the reactor on the basis of the reaction temperature.
According to the present invention, by measuring the educt temperatures and the momentary reaction temperature, the O2/C ratio present in the reactor or the deviation from a desired O2/C ratio is determined for an optimal hydrogen yield. The desired O2/C ratio is adjusted by controlling the mass flow rate of an educt, which in the following will also be called a controlled educt, with a defined known mass flow rate of the other educts. The known mass flow rates may be (adjustably) fixed, for example, by corresponding flow control devices, valves or also the rotational speed of a compressor. By the determined educt temperatures, the desired O2/C ratio at which an optimal hydrogen yield is achieved, is set in a (first) rough approximation. This takes place on the basis of comparative values which contain the flow amounts of the controlled educt or the control value for the controlled educt at different educt temperatures.
A correction of the approximate adjustment or a regulating/controlling of the desired O2/C ratio is carried out by the deviation of the actual reaction temperature from the desired reaction temperature as well as by additional comparative values which contain the flow amounts or the changes of the flow amounts of the controlled educt at different reaction temperature deviations. The correction until the desired O2/C ratio has been reached or the setting of the desired O2/C ratio, which depends on (1) the deviation of the reaction temperature, (2) the desired precision of the O2/C ratio, and (3) the quality of the comparative values, may require one or several correcting steps. The desired O2/C ratio is therefore adjusted or maintained by a fundamental/basic setting of an O2/C ratio according to the educt temperatures and a precise adjustment according to the reaction temperature.
The measuring of the educt temperatures and of the reaction temperature can take place simultaneously or successively in a timed or continuous manner. The adjusting of the mass flow rate of the controlled educt can also take place by a superposition of the adjustment according to the educt temperatures as well as the adjustment according to the actual reaction temperature (that is, in the manner of two control/automatic control circuits, for example, in the form of two controllers for the controlled educt). However, it is also possible to process the determined temperatures to obtain a control value, for example, by calibrating or weighting the determined temperatures or the temperature deviation and provide them as an input quantity for a single controller.
The comparative values are determined by simulation computations or experimentally and may be filed as a function, a characteristic curve, or a truth table. The comparative values can be stored, for example, in computer devices/memories or can be implemented implicitly in the form of a PID controller (proportional integral differential controller).
In an advantageous manner, the controlling and checking of the O2/C ratio and thus of the flow amount of the controlled educt is based on the determination of the reaction temperature and the educt temperatures. The determination of temperatures is constructively simpler than high-expenditure flow quantity determinations. An explicit measuring of the flow amounts is not necessary in this case. Control devices are required only for the educt to be controlled, either for the hydrocarbon-containing educt or the oxygen-containing educt. The method according to the present invention is particularly suitable for a static system, in which the amount of the hydrogen to be generated should essentially remain the same. This hydrogen quantity is defined by a corresponding selection of the mass flow rate of the known indicated educt.
For a gas generating system according to the present invention, the reactor is thermally insulated with respect to the environment to permit an approximately adiabatic operation. Devices are provided for measuring a momentary reaction temperature and for measuring the temperature of the educts. Computer devices are provided for analyzing the measured values and computing control values for the devices for controlling the mass flow rate of at least one educt.
An adiabatic reactor can be implemented without major expenditures by a thermal insulation with the suitable materials. The monitoring of educt flows takes place indirectly by temperatures sensors. Temperature sensors are not only cost-effective, but also are reliable in their operation and easy to handle.
In a further development of the method according to the present invention, a correction value ΔO2/C will then be determined. A corrected O2/C ratio will be adjusted when the temperature difference is larger than a defined value. By way of the amount of the defined value, the adjusting precision of the desired O2/C ratio can be determined.
In an advantageous further development of the method according to the present invention, in the case of the adjusting of the O2/C ratio, the mass flow rate of the hydrocarbon-containing educt is monitored by controlling the mass flow rate of the oxygen-containing educt, or the mass flow rate of the oxygen-containing educt is monitored by controlling the mass flow rate of the hydrocarbon-containing educt. For a change of the mass flow rate of the monitored educt, the O2/C ratio is maintained by the follow-up of the mass flow rate of the controlled educt. The monitoring or determination of the mass flow rate of the educt which is not controlled in each case (a monitored educt) permits a greater flexibility during the adjustment of the O2/C ratio. In particular, the hydrogen quantity to be generated per time unit can be adjusted in a controlled manner while maintaining the desired optimal O2/C ratio by changing the mass flow rate of the monitored educt. Thus, the method according to the present invention is specifically suitable for dynamic systems.
Although changes in the mass flow rate of the monitored educt can be determined and compensated by changes of the reaction temperature, this automatic control or control reacts relatively sluggishly to rapid changes. Particularly when controlling the O2/C ratio by the mass flow rate of the hydrocarbon-containing educt (of the fuel), load jumps when generating the hydrogen-containing synthesis gas result in an abrupt change of the mass flow rate of the oxygen-containing educt (normally of the supplied air mass). In the case of a load jump to a higher performance (increase of the air mass flow rate), a deviation of the O2/C ratio is caused in the direction toward higher temperatures. Because partial oxidation and autothermal reforming take place at very high temperatures, this may lead to a high thermal stressing of the components which may result in a possible failure. Because of the readjusting of the fuel mass flow rate at a point in time at which no great temperature change has yet occurred in the reactor, high temperature peaks are prevented, as may occur in the case of a pure temperature control. Particularly the controlling of the fuel quantity by a superimposing of the two control mechanisms permits a secure and optimal (dynamic) operation of the gas generating system.
In the case of an embodiment according to the present invention, the ratio of water to hydrocarbon-containing educt (S/C-ratio) during a change of the mass flow rate of the hydrocarbon-containing educts is maintained by the follow-up of the mass flow rate of the water. It was found that the ratio of water to the hydrocarbon-containing educt affects the hydrogen yield, although to a lesser extent than the O2/C ratio. Although a larger amount of water results in a greater hydrogen yield, the more water is metered, the more energy is required for the vaporization and overheating. Since no arbitrarily large amount of energy is available in the system, the preheating temperature of the educts will fall as the water metering rises. When the feeding of water is too large, the hydrogen yield is reduced because the educt preheating temperature is too low. An S/C ratio value of 1.5 was found to be advantageous. Thus, when the fuel mass flow rate is changed, the water quantity is correspondingly readjusted.
In a preferred embodiment of the method according to the present invention, PID controllers are used for controlling the mass flow rates of the educts. The educt temperatures, the reaction temperature, and optionally values for the mass flow rate of the monitored educt, and in the case of a PID controller for controlling the water quantity, values for the mass flow rate of the hydrocarbon-containing educt, are used as input quantities for the PID controllers. By the PID controllers, the educt mass flow rates can be controlled or automatically controlled in a simple manner. The comparative values in the PID controllers are implicitly implemented in the form of the proportionality between the input quantity and the control variable. Optionally, the measured values for the mass flow rates, the temperature, etc. can first be processed in computer devices before they are fed to the PID controller as input quantities.
In a further embodiment according to the present invention, only the temperature of the educts is determined whose temperature influence on the reaction temperature should not be neglected with respect to the desired adjusting precision of the O2/C ratio. Partial oxidation or autothermal reforming takes place at high temperatures. In this case, the reaction temperature, depending on the initial temperature of the educts, is influenced by these to a varying degree. Normally, the oxygen-containing educt component is preheated, while the fuel is fed to the reactor without preheating. When the temperature difference is correspondingly large, the influence of the fuel temperature on the reaction temperature will be negligible. As a result of the expedient limitation, the constructive and measuring-related expenditures can be further reduced.
In another embodiment of the present invention, a plurality of educts are mixed with one another before they are charged into the reactor and the temperature of this educt mixture is determined. As a result, the constructive and procedural measuring expenditures can be further reduced. For example, in the case of the autothermal reforming, the combining of the oxygen-containing component, preferably of the supplied air, with water is expedient so that only the temperature of the water vapor/air mixture is determined.
In another embodiment of the present invention, liquid fuel is used as a carbon-containing educt, and the setting of the desired O2/C ratio takes place by regulating the mass flow rate of the liquid fuel. The metering of liquids can basically be carried out in a simpler and significantly faster manner than that of gases so that a faster reaction time of the system can be implemented while the construction of the control devices is simultaneously simpler. The result is that optionally the mass flow rate of the oxygen-containing educt, normally supplied air, is monitored. Thus, only temperature sensors and an air flow rate sensor as well as computer and control devices are required for controlling the flow quantity of fuel and water.
In an embodiment of the present invention, the momentary reaction quantity of the reactor is determined by measuring the temperature of the catalyst surface and/or the temperature of the product gases. Both methods open up a reasonable access to the reaction temperature while the precision is sufficient. The temperature of the catalyst surface is generally slightly higher than the temperature of the product gases. However, this can easily be taken into account when computing the temperature difference between the actual and the desired reaction temperature.
In a preferred embodiment of the gas generating system, the devices for feeding and controlling the mass flow rate of the liquid fuel comprise an injection valve and a pressure control valve. The mass flow rate is controlled by the opening time and/or the pressure of the injection valve and can be adjusted in a fast and precise manner. Injection systems are standard applications in automobile engine technology and can be reliably operated on the basis of long-term experiences.
In another preferred embodiment of the present invention, a heat exchanger is provided for preheating the educts. The hydrogen yield is influenced by the preheating temperature of the educts. By way of the heat exchanger, the heat of the product gases can advantageously be used for preheating the educts.
In another embodiment of the present invention, the heat exchanger is designed such that, for the autothermal reforming, the oxygen-containing educt and water in the heat exchanger are mixed, are jointly heated, and are supplied to the reactor as mixed gas. Only a single device is required for preheating the educts. In addition, as a result of the mixing and the joint heating of the educts, the determination of the educt temperatures can be reduced to the individual temperature measuring of the mixed gas.
It is understood that the above-mentioned advantages and those which will be explained in the following can be used not only in the respective indicated combination but also in other combinations or alone without leaving the scope of the present invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the present invention when considered in conjunction with the accompanying drawings.